Embodiments of the invention relate generally to power electronics for injector heaters and more particularly to power electronics for control and monitoring of heater drivers for variable spray fuel injectors.
There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to minimize engine crank times and time from key-on to drive-away, while maintaining maximum fuel economy. These pressures apply to engines fueled with alternative fuels, such as ethanol, as well as to those fueled with gasoline.
During cold temperature engine start, the conventional spark ignition internal combustion engine is characterized by high hydrocarbon emissions and poor fuel ignition and combustibility. Unless the engine is already at a high temperature after stop and hot-soak, the crank time may be excessive, or the engine may not start at all. At higher speeds and loads, the operating temperature increases and fuel atomization and mixing improve.
During an actual engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions. The worst emissions are during the first few minutes of engine operation, after which the catalyst and engine approach operating temperature. Regarding ethanol fueled vehicles, as the ethanol percentage of the fuel increases to 100%, the ability to cold start becomes increasingly diminished, leading some manufacturers to include a dual fuel system in which engine start is fueled with conventional gasoline, and engine running is fueled with the ethanol grade. Such systems are expensive and redundant.
Another solution to cold start emissions and starting difficulty at low temperature is to pre-heat the fuel to a temperature where the fuel vaporizes quickly, or vaporizes immediately (“flash boils”), when released to manifold or atmospheric pressure. Pre-heating the fuel replicates a hot engine as far as fuel state is considered.
A number of pre-heating methods have been proposed, most of which involve preheating in a fuel injector. Fuel injectors are widely used for metering fuel into the intake manifold or cylinders of automotive engines. Fuel injectors typically comprise a housing containing a volume of pressurized fuel, a fuel inlet portion, a nozzle portion containing a needle valve, and an electromechanical actuator such as an electromagnetic solenoid, a piezoelectric actuator, or another mechanism for actuating the needle valve. When the needle valve is actuated, the pressurized fuel sprays out through an orifice in the valve seat and into the engine.
One technique that has been used in preheating fuel is to resistively heat metallic elements of the fuel injector with a time-varying or steady state electrical current. The electrical energy is converted to heat inside a component suitable in geometry and material to be heated by the Joule or Ohm losses that are caused by the flow of current through that component.
The heated fuel injector is useful not only in solving the above-described problems associated with gasoline systems, but is also useful in pre-heating ethanol grade fuels to accomplish successful starting without a redundant gasoline fuel system.
Because the heating technique uses an electrical current, the system includes electronics for providing an appropriate excitation to the component in the fuel injector. This excitation may include controlling the electrical energy and determining when that electrical energy is applied.
Conventional resistive heating is accomplished open-loop, or without control of electrical energy based on a temperature. A remote thermostat or computational model may be incorporated to provide some control to prevent a runaway temperature event and damage to the fuel injector. More sophisticated methods may monitor the current through the heater to estimate the temperature or direct thermocouple, positive/negative temperature coefficient sensor, or other means for determining the temperature for a more precise regulation of injector heater temperature.
The metallic component that is heated will have a positive temperature coefficient of resistance to electrical current (i.e., its electrical resistance will increase as its temperature increases). Ideally, knowing the initial resistance and final resistance would allow the temperature of the component to be known with some degree of precision. The best metals for resistive heaters usually have very small positive temperature coefficients and therefore measurement of the change in resistance by only monitoring current will be desensitized by harness resistance and aging of numerous interconnecting components. Therefore, it becomes difficult to distinguish a change in resistance of the heater component from a change in resistance of other components connected in series.
It would be advantageous to more precisely know the resistance change of the heater component such that control of the temperature may be accomplished.